# Allow for periodic boundaries which are connected by a rotation.

Pervious implementation for periodic boundary conditions only allowed for periodic boundaries that were a translation of each other. This fix allows for the boundaries to be connected by a rotation. This is achieved by specifying an axis of rotation and rotation angle. For example in NekMesh one would specify a rotation about the 'z' axis as:

NekMesh -m peralign:rot="-PI/3.0":dir=z:tol=1e-6:surf1=5:surf2=4

In 3D this information is also required in the boundary conditions so that periodic faces can be appropriately aligned. So for example for a rotation about x of an angle PI/11 you would have to specify

` <P VAR="u" USERDEFINEDTYPE="Rotated:x:PI/11:1e-6" VALUE="[2]" />`